Unlike most Unix systems and services, Windows does not require Python natively
and thus does not pre-install a version of Python. However, the CPython team
has compiled Windows installers (MSI packages) with every release for many years.

With ongoing development of Python, some platforms that used to be supported
earlier are no longer supported (due to the lack of users or developers).
Check PEP 11 for details on all unsupported platforms.

Windows has a built-in dialog for changing environment variables (following
guide applies to XP classical view): Right-click the icon for your machine
(usually located on your Desktop and called “My Computer”) and choose
Properties there. Then, open the Advanced tab
and click the Environment Variables button.

In short, your path is:

My Computer
‣ Properties
‣ Advanced
‣ Environment Variables

In this dialog, you can add or modify User and System variables. To change
System variables, you need non-restricted access to your machine
(i.e. Administrator rights).

Another way of adding variables to your environment is using the set
command:

set PYTHONPATH=%PYTHONPATH%;C:\My_python_lib

To make this setting permanent, you could add the corresponding command line to
your autoexec.bat. msconfig is a graphical interface to this
file.

Viewing environment variables can also be done more straight-forward: The
command prompt will expand strings wrapped into percent signs automatically:

Besides using the automatically created start menu entry for the Python
interpreter, you might want to start Python in the command prompt. As of
Python 3.3, the installer has an option to set that up for you.

At the “Customize Python 3.3” screen, an option called
“Add python.exe to search path” can be enabled to have the installer place
your installation into the %PATH%. This allows you to type
python to run the interpreter. Thus, you can also execute your
scripts with command line options, see Command line documentation.

If you don’t enable this option at install time, you can always re-run the
installer to choose it.

The alternative is manually modifying the %PATH% using the
directions in Excursus: Setting environment variables. You need to set your %PATH%
environment variable to include the directory of your Python distribution,
delimited by a semicolon from other entries. An example variable could look
like this (assuming the first two entries are Windows’ default):

Python usually stores its library (and thereby your site-packages folder) in the
installation directory. So, if you had installed Python to
C:\Python\, the default library would reside in
C:\Python\Lib\ and third-party modules should be stored in
C:\Python\Lib\site-packages\.

An empty entry is added at the start, which corresponds to the current
directory.

If the environment variable PYTHONPATH exists, as described in
Environment variables, its entries are added next. Note that on Windows,
paths in this variable must be separated by semicolons, to distinguish them
from the colon used in drive identifiers (C:\ etc.).

Additional “application paths” can be added in the registry as subkeys of
\SOFTWARE\Python\PythonCore\version\PythonPath under both the
HKEY_CURRENT_USER and HKEY_LOCAL_MACHINE hives. Subkeys which have
semicolon-delimited path strings as their default value will cause each path
to be added to sys.path. (Note that all known installers only use
HKLM, so HKCU is typically empty.)

If the environment variable PYTHONHOME is set, it is assumed as
“Python Home”. Otherwise, the path of the main Python executable is used to
locate a “landmark file” (Lib\os.py) to deduce the “Python Home”. If a
Python home is found, the relevant sub-directories added to sys.path
(Lib, plat-win, etc) are based on that folder. Otherwise, the core
Python path is constructed from the PythonPath stored in the registry.

If the Python Home cannot be located, no PYTHONPATH is specified in
the environment, and no registry entries can be found, a default path with
relative entries is used (e.g. .\Lib;.\plat-win, etc).

The end result of all this is:

When running python.exe, or any other .exe in the main Python
directory (either an installed version, or directly from the PCbuild
directory), the core path is deduced, and the core paths in the registry are
ignored. Other “application paths” in the registry are always read.

When Python is hosted in another .exe (different directory, embedded via COM,
etc), the “Python Home” will not be deduced, so the core path from the
registry is used. Other “application paths” in the registry are always read.

If Python can’t find its home and there is no registry (eg, frozen .exe, some
very strange installation setup) you get a path with some default, but
relative, paths.

Without the Python launcher installed, Python scripts (files with the extension
.py) will be executed by python.exe by default. This executable
opens a terminal, which stays open even if the program uses a GUI. If you do
not want this to happen, use the extension .pyw which will cause the script
to be executed by pythonw.exe by default (both executables are
located in the top-level of your Python installation directory). This
suppresses the terminal window on startup.

You can also make all .py scripts execute with pythonw.exe,
setting this through the usual facilities, for example (might require
administrative rights):

The Python launcher for Windows is a utility which aids in the location and
execution of different Python versions. It allows scripts (or the
command-line) to indicate a preference for a specific Python version, and
will locate and execute that version.

You should notice the version number of your latest Python 2.x installation
is printed. Now try changing the first line to be:

#! python3

Re-executing the command should now print the latest Python 3.x information.
As with the above command-line examples, you can specify a more explicit
version qualifier. Assuming you have Python 2.6 installed, try changing the
first line to #!python2.6 and you should find the 2.6 version
information printed.

The launcher should have been associated with Python files (i.e. .py,
.pyw, .pyc, .pyo files) when it was installed. This means that
when you double-click on one of these files from Windows explorer the launcher
will be used, and therefore you can use the same facilities described above to
have the script specify the version which should be used.

The key benefit of this is that a single launcher can support multiple Python
versions at the same time depending on the contents of the first line.

If the first line of a script file starts with #!, it is known as a
“shebang” line. Linux and other Unix like operating systems have native
support for such lines and are commonly used on such systems to indicate how
a script should be executed. This launcher allows the same facilities to be
using with Python scripts on Windows and the examples above demonstrate their
use.

To allow shebang lines in Python scripts to be portable between Unix and
Windows, this launcher supports a number of ‘virtual’ commands to specify
which interpreter to use. The supported virtual commands are:

/usr/bin/envpython

/usr/bin/python

/usr/local/bin/python

python

For example, if the first line of your script starts with

#! /usr/bin/python

The default Python will be located and used. As many Python scripts written
to work on Unix will already have this line, you should find these scripts can
be used by the launcher without modification. If you are writing a new script
on Windows which you hope will be useful on Unix, you should use one of the
shebang lines starting with /usr.

Two .ini files will be searched by the launcher - py.ini in the
current user’s “application data” directory (i.e. the directory returned
by calling the Windows function SHGetFolderPath with CSIDL_LOCAL_APPDATA)
and py.ini in the same directory as the launcher. The same .ini
files are used for both the ‘console’ version of the launcher (i.e.
py.exe) and for the ‘windows’ version (i.e. pyw.exe)

Customization specified in the “application directory” will have
precedence over the one next to the executable, so a user, who may not
have write access to the .ini file next to the launcher, can override
commands in that global .ini file)

In some cases, a version qualifier can be included in a command to dictate
which version of Python will be used by the command. A version qualifier
starts with a major version number and can optionally be followed by a period
(‘.’) and a minor version specifier. If the minor qualifier is specified, it
may optionally be followed by “-32” to indicate the 32-bit implementation of
that version be used.

For example, a shebang line of #!python has no version qualifier, while
#!python3 has a version qualifier which specifies only a major version.

If no version qualifiers are found in a command, the environment variable
PY_PYTHON can be set to specify the default version qualifier - the default
value is “2”. Note this value could specify just a major version (e.g. “2”) or
a major.minor qualifier (e.g. “2.6”), or even major.minor-32.

If no minor version qualifiers are found, the environment variable
PY_PYTHON{major} (where {major} is the current major version qualifier
as determined above) can be set to specify the full version. If no such option
is found, the launcher will enumerate the installed Python versions and use
the latest minor release found for the major version, which is likely,
although not guaranteed, to be the most recently installed version in that
family.

On 64-bit Windows with both 32-bit and 64-bit implementations of the same
(major.minor) Python version installed, the 64-bit version will always be
preferred. This will be true for both 32-bit and 64-bit implementations of the
launcher - a 32-bit launcher will prefer to execute a 64-bit Python installation
of the specified version if available. This is so the behavior of the launcher
can be predicted knowing only what versions are installed on the PC and
without regard to the order in which they were installed (i.e., without knowing
whether a 32 or 64-bit version of Python and corresponding launcher was
installed last). As noted above, an optional “-32” suffix can be used on a
version specifier to change this behaviour.

Examples:

If no relevant options are set, the commands python and
python2 will use the latest Python 2.x version installed and
the command python3 will use the latest Python 3.x installed.

The commands python3.1 and python2.7 will not consult any
options at all as the versions are fully specified.

If PY_PYTHON=3, the commands python and python3 will both use
the latest installed Python 3 version.

If PY_PYTHON=3.1-32, the command python will use the 32-bit
implementation of 3.1 whereas the command python3 will use the latest
installed Python (PY_PYTHON was not considered at all as a major
version was specified.)

If PY_PYTHON=3 and PY_PYTHON3=3.1, the commands
python and python3 will both use specifically 3.1

In addition to environment variables, the same settings can be configured
in the .INI file used by the launcher. The section in the INI file is
called [defaults] and the key name will be the same as the
environment variables without the leading PY_ prefix (and note that
the key names in the INI file are case insensitive.) The contents of
an environment variable will override things specified in the INI file.

For example:

Setting PY_PYTHON=3.1 is equivalent to the INI file containing:

[defaults]
python=3.1

Setting PY_PYTHON=3 and PY_PYTHON3=3.1 is equivalent to the INI file
containing:

If an environment variable PYLAUNCH_DEBUG is set (to any value), the
launcher will print diagnostic information to stderr (i.e. to the console).
While this information manages to be simultaneously verbose and terse, it
should allow you to see what versions of Python were located, why a
particular version was chosen and the exact command-line used to execute the
target Python.

Even though Python aims to be portable among all platforms, there are features
that are unique to Windows. A couple of modules, both in the standard library
and external, and snippets exist to use these features.

If you want to compile CPython yourself, first thing you should do is get the
source. You can download either the
latest release’s source or just grab a fresh checkout.

The source tree contains a build solution and project files for Microsoft
Visual C++, which is the compiler used to build the official Python releases.
View the readme.txt in their respective directories:

Directory

MSVC version

Visual Studio version

PC/VS9.0/

9.0

2008

PCbuild/

10.0

2010

Note that any build directories within the PC directory are not
necessarily fully supported. The PCbuild directory contains the files
for the compiler used to build the official release.

Check PCbuild/readme.txt for general information on the build process.

or “Creating Python extensions in C/C++ with SWIG and compiling them with
MinGW gcc under Windows” or “Installing Python extension with distutils
and without Microsoft Visual C++” by Sébastien Sauvage, 2003